Concentrative uridine transport by murine splenocytes: kinetics, substrate specificity, and sodium dependency

A previous report from this laboratory indicated that the concentration of free uridine (Urd) in many normal murine tissues greatly exceeds that in plasma. We now report that Urd uptake by isolated murine splenocytes is concentrative, and that the rate of uptake from medium containing 10 to 500 microM [3H]Urd conforms to a process that is saturable with a Km of 38.0 +/- 4.1 (SE) microM and Vmax of 2.70 +/- 0.27 pmol/s/microliter cell water. Other ribosyl and deoxyribosyl pyrimidine nucleosides or their analogues were not concentrated by splenocytes; however, ribosyl and deoxyribosyl purine nucleosides and, to a lesser extent, deoxyuridine did inhibit Urd uptake. In this system Urd uptake was not inhibited by 1 microM nitrobenzylthioinosine or 10 microM dipyridamole but was significantly inhibited by 5 mM NaN3 or 250 microM KCN. Transport of Urd involves Na+ cotransport as evidenced by complete inhibition when Na+ is replaced by Li+ in the incubation medium, and it is also inhibited by 3 mM ouabain. Active Urd transport coexists with the nonspecific, carrier mediated, facilitated diffusion of nucleosides as demonstrated by the inhibition of Urd efflux and thymidine influx in splenocytes by nitrobenzylthioinosine and dipyridamole. Under identical conditions, Urd entry into L1210 leukemia cells was nonconcentrative and non-Na+ dependent but inhibited by nitrobenzylthionosine. That nucleosides enter most cultured neoplastic cell lines by facilitated diffusion and not the active transport mechanism for Urd confirms earlier findings and may represent an exploitable target for chemotherapy.